Fuel cell technology shows great promise as an alternative energy source for diverse applications. Fuel cells convert chemical energy released during an electrochemical reaction in a reaction chamber which includes a positive electrode or cathode, a negative electrode or anode, an electrolyte which transports electrically charged particles from one electrode to the other, a catalyst which accelerates the chemical reactions at the electrodes and a fuel. The key element of fuel cell operation regardless of the type is the flow of the appropriate ions between the cathode and the anode. If free electrons or other substances pass through the electrolyte, they may foul the catalyst, collect on the electrodes or otherwise disrupt the chemical reaction, thereby causing the power output of the fuel cell to fall off or stop completely.
Various types of fuel cells have been developed and are known generally by their function, structure or the fuel source. Examples include proton exchange membrane (PEM) fuel cells, also known as polymer electrolyte membrane fuel cells (PEMFC), direct methanol fuel cells (DMFC), direct alcohol fuel cells (DAFC), and microbial fuel cells (MFC), to name a few. Fuel cells are of particular interest in the renewable energy field inasmuch as they present potential sources of power which do not rely upon fossil fuels in their operation.
Renewable sources of energy are critical to reduce the impact of global warming and to meet the world's future energy demands. A sustainable and renewable energy source will preserve the environment and decrease dependence on foreign sources of oil. However, finding an economical and efficient way to produce energy from a renewable source has been elusive. Energy produced from biomass can meet this critical challenge, and its ubiquitous nature makes biomass attractive for widespread use as a fuel, provided that energy production based thereon as a fuel source may be accomplished in an economical and efficient manner. However, thus far, technologies which have been developed to convert biomass into energy have proven to be either inefficient or uneconomical, and the sustainable power output level of fuel cells in general has been disappointing.
For example, Halme et al., in European Patent Application No. EP 1 376 729 A2 for Biocatalytic Direct Alcohol Fuel Cell published on Jun. 13, 2003 (the '729 application), disclose a biocatalytic fuel cell which addresses the problems associated with limited power output in both hybrid fuel cells, where one of the electrodes is substantially chemical and the other substantially biocatalytic, and completely biocatalytic fuel cells. These problems stem from the mild operating conditions necessary to sustain the enzymatic catalyst, a live microorganism, in either the anode or the cathode reaction chamber. As noted in the Halme disclosure, chemical, direct-acting fuel cells require intensive reaction conditions such as high temperatures and strongly acidic or alkaline solutions as well as an expensive platinum secondary catalyst to achieve any appreciable power output. These conditions are not conducive to the survivability of the enzymatic catalyst. Halme and his colleagues address this problem by providing a direct alcohol fuel cell which has an anode chamber and one or more cathode chambers and uses a so-called “triplet” in which a biocatalyst oxidizes fuel in the anode chamber and cooperates with a mediator to convey the generated electrons to a current collector electrode. Oxygen or corresponding oxidants are reduced in the cathode chamber by means of electrons originating from a current donor electrode and a chemical or biocatalyst (or a combination thereof). While the '729 application discusses certain prior art references to the use of human fluids or plant sap as the fuel source, alcohol is the fuel required in the operation of the disclosed system.
More recently, U.S. Patent Application Publication No. 2008/0274393 A1 for Hydrogel Barrier for Fuel Cells published by Markoski et al. on Nov. 6, 2008, (the '393 application”) discloses a fuel cell that includes a system for reducing the amount of water at or within the cathode and to prevent the occurrence of “fuel crossover”, both of which reduce the electrical output of the fuel cell. “Fuel crossover” is a situation where the fuel crosses through the membrane in the cell designed to separate the anode from the cathode and reacts with the catalyst directly in the presence of oxygen to generate heat, water and carbon dioxide but no useable electric current. While the Markoski et al. application discloses the use of an aqueous liquid and a polymer positioned intermediate the anode and the cathode to maintain the separation of liquids surrounding the respective electrodes (in this case methanol and water) in a direct methanol fuel cell (DMFC), limited output is obtained from a non-biomass fuel source.
The use of biomass, material derived from plants, is a highly attractive alternative to fossil fuels. The direct conversion of biomass to electricity is receiving considerable attention due to improved overall efficiencies and reduced generation of greenhouse gaseous byproducts. Although biomass is readily available everywhere, extensive processing is required before it can be used in any modern system to extract power. An example is the combustion of biomass to convert water to steam, which then performs mechanical work to spin a turbine to generate electrical power. It is a three-step process: combustion of biomass in boilers to generate steam, conversion of the heat energy in steam to mechanical work in turbines, and finally, use of rotary generators to produce electricity. Thermodynamic losses during the three conversion steps limit the overall efficiency of conversion to a range of approximately 20% to 40%; although, higher efficiencies may be achievable for large systems (>100 MW of electrical power).
Other known processes for the derivation of energy from biomass include gasification, pyrolysis, and anaerobic digestion. Gasification can be up to 50% efficient when combined with a heating recovery system but is uneconomical due to elaborate process requirements. Pyrolysis, the conversion of biomass to fuel only, can be up to 80% efficient, but the overall conversion efficiency to electricity is less than 20%. Distillation processes, another example, can consume about a third of the total chemical energy in the biofuel. If the energy lost during distillation can be avoided and the biofuel can be converted to electricity at efficiencies of 40%-50% in fuel cells, then direct conversion of fermenting biomass to electricity becomes a compelling option. Even gas-to-electricity generators, which use biogas directly, are only 30% efficient, exclusive of the losses in the conversion of biomass to biogas. Conversion efficiencies are even lower when wet biomass with high water content is used due to the additional energy required to drive out the moisture. The existing processes also release environmentally harmful gases, including ammonia, carbon monoxide, formaldehyde, nitrogen oxides, hydrocarbons, and sulfur oxides. Anaerobic digestion is only 10% to 16% efficient when electricity is the desired end product, but wet biomass with higher water content may be used. It also results in the production of unpleasant and harmful gases, making it unacceptable for wider deployment.
In each of the aforementioned technologies, direct electrical power from biomass is not produced, and the overall efficiency is drastically reduced due to intermediate energy conversion processes that must be used. Currently, it is more cost effective to burn fossil fuels to produce electricity than to use biomass in the form of waste or plant matter. Therefore, the key to making biomass and derived biofuels a compelling source of electrical power with wide acceptance is to produce power directly from biomass with minimal processing, high efficiency, low cost, and minimal harmful emissions.
One technology capable of producing electrical power directly from biomass with little to no preprocessing is a fuel cell. The potential of directly converting biological fuels to electrical energy using inexpensive, self-sustaining microbial catalysts make Microbial Fuel Cells (MFCs) attractive for persistent energy harvesting. Another key advantage for biomass electro-oxidation, especially using proton exchange membrane (PEM) fuel cells, is the generation of valuable co-products that can enable the economics of the overall process. For example, while research is continually driving to more efficient fuel cell catalysts in direct ethanol fuel cells (DEFCs), residual acetic acid could still be a useful commodity (i.e., in de-icing).
Typically, a significant amount of processing is required before biofuels can be utilized in fuel cells. For example, fermentation is often used to produce ethanol, which, as noted above, when combusted to produce electricity, is a highly inefficient process due to intermediate steps involving other types of energy. This processing includes purification steps to eliminate foulants in the fuel, which can kill the fuel cell power output in a relatively short time. Moreover, pure fuels (such as neat ethanol) are not used in PEM fuel cells due to large crossover-related losses, which Markoski et al. attempt to address in their work disclosed in the '393 application discussed above. Dilution is necessary and a fuel delivery system is needed, which adds to the size, parasitic energy requirement, and the cost of the overall system.
Direct conversion of biomass to electricity without feed purification or dilution can be achieved using a microbial fuel cell (MFC). Allen and Bennetto showed that MFCs utilizing mediators (i.e., 2-hydroxy-1,4-naphthoquinone) to shuttle electrons from Proteus vulgaris to the anode from culture broth generate ˜10 μW cm−2 power at coulombic efficiencies of ˜30%. Allen, R. and Bennetto, P., Microbial Fuel Cells, Appl. Biochem. Biotechnology, 39-40, 27-44 (1993). Chaudhuri and Lovley showed that MFCs utilizing anode respiring bacteria, which do not require mediators, generate <10 μW cm−2 at coulombic efficiencies>80% via direct electron transfer. Chaudhuri, S. K., Lovley, D. R., Electricity Generation by Direct Oxidation of Glucose in Mediatorless Microbial Fuel Cells, Nature Biotechnology 21, 1229-1232 (2003). Although much work has been done to increase the power densities of such MFCs, power output levels in excess of approximately 100 μW cm−2 have not yet been achieved.
Direct utilization of products from microbial metabolism in DEFCs to generate approximately 900 μW cm−2 in a single step has been reported. Mackie, D. M., Liu, S., Benyamin, M., Ganguli, R., Sumner, J. J., Direct Utilization of Fermentation Products in an Alcohol Fuel Cell, J. Power Sources 232, 34-41 (2013). Although the coulombic efficiency of these biofuel cells are limited, since conversion of sugars were limited only to acetic acid and not completely to carbon dioxide, the much higher power densities may enable practical applications and represent an exciting step forward. A critical technical challenge is the design of effective microbial growth media to minimize the presence of organics, salts, proteins, and other species in the fermentation broth that interfere with, or foul, the catalyst or the membrane electrode assembly (MEA). Consequently, high power densities achieved initially declined over time. However, if foulants could be separated from the products of microbial metabolism to prevent power decline, then the conversion of simple sugars to electricity would be compelling.
In view of the foregoing, it will be apparent to those skilled in the art from this disclosure that a need exists for an improved reverse osmosis (RO) membrane-integrated DEFC that (a) eliminates the fouling problem of such fuel cells, allowing the use of dirty biomass as fuel, and (b) minimizes crossover-related losses by passively regulating the concentration of ethanol in the DEFC. Moreover, a need exists for an improved method of generating electrical power in fuel cells using biomass fuel sources without the need for intermediate, efficiency-reducing processing steps associated with the conversion of biomass into suitable fuels.